This invention relates to a cobalt-based alloy containing a substantial proportion of chromium and referred to herein as a cobalt-chromium superalloy. More particularly the invention is concerned with certain cobalt-chromium oxide dispersion strengthened (O.D.S.) superalloys produced by a mechanical alloying process.
The term "superalloy" is a term of art which generally signifies an alloy having particularly high strength, good mechanical and corrosion-resistant characteristics and a stable microstructure. Of particular interest are those alloys which additionally retain high strength properties (and stable microstructures) following thermal treatments at extremely high temperatures.
The known alloy Vitallium.RTM. is a high corrosion-resistant cobalt/chromium alloy which is used successfully in numerous orthopaedic applications. A typical composition for Vitallium is the following:
______________________________________ Element % by weight ______________________________________ Carbon 0.25 Silicon 0.75 Manganese 0.70 Chromium 28.00 Molybdenum 5.50 Cobalt 64.80 ______________________________________
Because of its many favorable properties, particularly corrosion resistance, Vitallium is used extensively in orthopaedic applications, especially for prostheses. A particularly useful development in the area of orthopaedic implants is the provision of a porous coating in the form of multiple layers of spherical Vitallium particles on the surface of a Vitallium implant to encourage bone-growth in a cementless system.
It is known that the properties of a given metal alloy are dependent upon its composition and also upon the manner in which the various alloying ingredients are formed into the final alloy. "Mechanical alloying" is a process which produces homogeneous composite particles with an intimately dispersed, uniform internal structure. The process is described in an article entitled "Dispersion Strengthened Superalloys by Mechanical Alloying" by John S. Benjamin, Metallurgical Transactions, Vol. 1, October 1970, p. 2943.
U.S. Pat. No. 3,591,362, issued July 6, 1971 to John S. Benjamin discloses a composite metal powder formed by the technique of mechanical alloying.
Mechanical alloying is particularly advantageous for making superalloys that cannot be made readily by melting or by conventional powder metallurgy. Since the strength, and certain other properties, of superalloys depends ultimately on the presence of dispersions of intermetallic compounds and the utility of the superalloy is inherently limited by the stability of these components, the choice of dispersant materials, as well as alloying metals, is important for the performance of the final alloy.
It is known that the inclusion of certain selected oxides in the alloy composition can improve the properties of the final alloy and oxide dispersion strengthened (O.D.S.) superalloys made by the mechanical alloying process exhibit high-temperature strength and stability as a result of the presence of stable oxide dispersions which resist thermal damage and permit much greater freedom in alloy design.
It has now been found that improved cobalt-chromium superalloys made in accordance with O.D.S. mechanical alloying procedures not only have the high corrosion-resistant properties typical of Vitallium but also have excellent room temperature strength (tensile and fatigue) properties which are substantially retained after exposure to severe thermal conditions. These properties are substantially more advantageous than would be expected from prior art alloys made by similar techniques, for example the nickel-chromium, cobalt-chromium (of different composition to those specified herein), and iron-chromium systems disclosed in U.S. Pat. No. 3,591,362.